How to copy a specific area in a 3d map?
Your request it's not so clear, but I'll try anyway.


press Tab to get into Edit Mode
press Num 7, Num 5 to get in Top ortographic view
press the X-Ray button (in the picture)
press B and drag with the mouse to select the areas you want to delete (or use the circle selection tool with C)
press Del (the "delete" button, which has different names in different countries) and choose Vertices





keep doing that until you get the area you want, and try to cut along streets or parks, so that you don't cut buildings in half
press Tab to get back into Object Mode
select your model and export it from the File > Export menu


If you want to make a game or something else and you need the map to be closed (so a smaller map instead of a portion of a bigger one), you need to create some natural barriers, like walls, mountains or sea, on the edges of the map.

While doing that you need to re-route the severed streets (just copy-paste the existing ones)